(1) Field of the Invention
The present invention relates to a process for preparing an oxide superconductor having a high critical current density. More particularly, the present invention relates to a process for preparing an oxide superconductor wherein fine RE.sub.2 BaCuO.sub.5 is homogeneously dispersed in a superconducting phase.
(2) Description of the Related Art
The discovery of an oxide superconductor of a YBaCuO system having a critical temperature (T.sub.c) exceeding 90K has enabled liquid nitrogen to be used as a cooling agent, and this has led to research into ways in which such a superconductor can be put to practical use. Currently, however, the oxide superconductor of the type described above has a low critical current density (J.sub.c), which is of the utmost importance from the viewpoint of practical use, and thus a practical use for the oxide superconductor in a liquid nitrogen atmosphere has not yet been found.
Recently, it has become possible to obtain a critical current density exceeding 10000 A/cm.sup.2, i.e., a critical current density close to a level necessary for practical use in a magnetic field as high as 1 tesla (T), through the formation of a superconducting phase from a molten state (see M. Murakami et al., Japanese Journal of Applied Physics, Vol. 28, 1989, p. 1189).
This method focuses on the phenomenon that a superconducting phase (YBa.sub.2 Cu.sub.3 O.sub.x) is formed by a peritectic reaction of Y.sub.2 BaCuO.sub.5 O.sub.x (hereinafter referred to as "211 phase") with a liquid phase, the growth of the superconducting phase is accelerated by means of finely and homogeneously dispersing a 211 phase in a liquid phase, and at the same time, a 211 phase capable of serving as a pinning center can be successfully dispersed in the superconducting phase. The 211 phase is homogeneously and finely dispersed by a treatment which comprises rapidly heating the system to a temperature as high as at least 1200.degree. C. and then quenching the system, to thereby finely disperse the Y.sub.2 O.sub.3 serving as a nucleus of the formation of the 211 phase.
When the above-described conventional means is used, however, there is a tendency for Y.sub.2 O.sub.3 to agglomerate and form a large mass when the system is heated to 1200.degree. C. or higher. Further, since Y.sub.2 O.sub.3 is heavier than the liquid phase, it sinks below the liquid phase, and thus it becomes difficult to homogeneously disperse the Y.sub.2 O.sub.3. Further, limitations on the shape of the quenched material have made it difficult to prepare a molding having a desired shape.
The present invention has been created with a view to solving the above-described problems.